The proposed research is directed towards understanding the molecular mechanisms regulating gene expression in a simple eukaryotic organism, the yeast Saccharomcyes cervesiae. In particular, we propose to contine our studies of a number of aspects of the yeast mating process and test our hypothesis for mating type interconversion. The mating process is controlled by the mating type locus, which can exist in one of two states, a or alpha. These alleles control the production of and response to mating type specific pheromones, alpha factor, and a new pheromone discovered in our laboratory. We propose to determine the manner in which the mating type locus regulates such mating specific functions by biochemical and genetic analysis of presently-existing mutants defective in mating. In particular, we propose to continue our studies of the structure of the alpha mating type locus, which appears to code for three functions. Although mating types a and alpha are stable to growth, rare interconversions from a to alpha and from alpha to a can be observed spontaneously. In addition, a gene (HO) exists which promotes this interconversion at high frequency. We have proposed that this interconversion occurs by insertion and removal of informational cassettes -- that is, blocs of mating type locus information -- into the mating type locus. We are testing this hypothesis by identification of mutations in these cassettes and examination of whether the mutations move from one chromosomal location to another. This study of control of the mating process in yeast will provide information concerning the molecular bases by which genes are controlled and by which a stable switch is established and maintained in a eukaryotic organism.